Add a .bazelversion file. (#11)

This locks in the Bazel version currently supported by this repo.

Part of https://github.com/material-components/material-components-ios/issues/9363

Note: bazel 1.0.0 support was already added in #6, so this PR is simply documenting that fact. Bazel 2.0.0 is equivalent in terms of Apple platform support.

Tested manually by running:

```
bazel test //...
```

.bazelversion

@@ -0,0 +1 @@
+2.0.0